In projection exposure apparatuses for semiconductor lithography, and in particular in the objectives used in the apparatuses, rapidly moved optical elements, for example mirrors, have long been used to correct image errors during the exposure. However, the movement or the acceleration of the optical elements generally means that, on account of the prevailing inertial forces, certain deformations of the optical elements involved occur during their movement or during their acceleration. In the past, however, in conventional projection exposure apparatuses, the deformations in question were often too small to lead to a problematic deterioration of the imaging properties of the system. More recently, however, systems have increasingly been used in which a relatively rapid movement of the mirrors is needed, particularly in view of the fact that every displacement of the mask used for the exposure leads to an astigmatic error, which can only be corrected via a movement of the mirror. In operation, however, the masks, or so-called reticles, are moved regularly.
In addition, higher exposure rates and low acceptable error budgets lead to a greater importance of the deformations of the wafer that is to be exposed. The wafer usually heats up during the operation of the apparatus, which results in a deformation that cannot be corrected on the wafer itself or that can only be corrected with very great effort on the wafer itself. For this reason too, future systems will need to meet increasing demands in respect of rapid positionability of the mirrors used. Moreover, as the numerical aperture of the objectives used increases, the mirror diameters also increase. Since the mirrors are usually mounted with kinematic determination at three points, there is therefore also an increased likelihood of the mirror sagging on account of an acceleration acting on it. In particular, this sagging is often proportional to the fourth power of the mirror diameter. With the same material and the same the thickness, a doubling of the mirror diameter consequently can lead to a sixteen times greater deformation which, depending on optical sensitivity, is manifested in a thirty-two times greater wavefront error.
A related set of issues also exists for further components of projection exposure apparatuses, which further components also experience deformations as a result of dynamic accelerations, with the deformations potentially leading overall to a deterioration in the performance of the apparatus.
The possibilities of overcoming these problems by changing the materials used can be extremely limited. The concepts noted above relate only to the compensating of gravitational effects and are not suitable for effectively reducing sagging under dynamic loads. A corresponding concept is disclosed in the German laid-open specification DE 10 2012 214 232 A1, which was filed by the applicant and which is fully incorporated herein by reference.